Recently, the advanced countries promote green energy based on the energy conservation and the environmental awareness. The white light emitting diode (LED) gradually replaces the traditional lighting because the white light emitting diode has the advantages of small volume, low electric power consumption, long lifetime up to 100,000 hours, low heat dissipation and short response time, and therefore the white light emitting diode can be applied to the miniaturization application equipment and also be operated under high frequency. The electricity consumption of the white light emitting diode is eighth to tenth of the conventional light bulb, and half of the fluorescent lamp. So, the thermal radiation of the white light emitting diode is also low when compared with the traditional lighting. The white light emitting diode has been considered as a new light source in the twenty-first century with the characteristics of energy saving and environmental protection.
The white light provided by using a blue light-emitting diode with a yellow phosphor is the mature technology in the industry. In 1996, Japan's Nichia (Nichia Chemical) developed a high-efficiency white light source by providing a series of yellow yttrium aluminum garnet phosphors (Y3Al5O12:Ce, YAG:Ce) with the indium gallium nitride (InGaN) based blue light-emitting diode. The white light LED adopts a complementary relation between the blue emission of the InGaN-based light-emitting diode and the yellow emission of the YAG-based phosphor, so the red emission is insufficient. Furthermore, the luminous efficiency of the YAG-based phosphor at high environment temperature decreases accompanied with the increase of environment temperature. Therefore, the CRI (Color Rendering Index) of the conventional white light LED is approximately 70 to 80, and is not high enough for many applications like lighting and display.